Due to further increased fraud and counterfeit of products and documents novel anticounterfeit measures are required more than ever. For many years holograms have been the preferred security-technology and many different generations of hologram-based security features have been fabricated. Meanwhile, this technology is more than 30 years old and therefore well known and widespread. This situation represents a security risk since many people have access to the hologram technology. With the availability of digital hologram printers, the path to easy to use holographic mastering systems have further increased. These printers allow the production of many different types of holograms and a minimal knowledge of holographic set-ups or laser writers is required. Such equipment allows the preparation of masters for the subsequent metal master fabrication and the replication into thin-films in large volumes. Therefore, due to the widespread possibilities of generating holographic structures, holography is losing importance in the field of security elements.
However, in the field of security devices there is a permanent need for new and different devices in order to have the possibility to reduce the risk of reproduction also by changing the type of security devices.
An example of such an approach is explained in the patent application WO-01/03945, where the optical effect is e.g. due to the combination of holographic or grating structures and a colourshifting multi-layer interference effect.
Furthermore anisotropic diffusers and reflectors have been proposed with patterned anisotropy in Ibn-Elhaj et al., “Optical Polymer Thin Films With Isotropic And Anisotropic Nano-Corrugated Surface topologies”, Nature, 2001, vol. 410, p. 796-799. For the making of the structures use is made of a so called monomer corrugation technology (MC), which relies on the fact that phase separation of a mixture or blend applied to a substrate is induced by crosslinking with exposure to UV radiation. The removal of non-crosslinked components leaves a structure with a specific surface topology comprising grooves, dips and/or pores. With suitably scaled surface topologies, directional light diffusers may be built. Among other substrates, the use of an aluminum mirror as a substrate is proposed, onto which a patterned MC-film is produced with the above UV-induced phase separation of the mixture and subsequent removal of non-cross-linked material. Thereby, instead of a directional transmission of the pattern, a directional reflector is made available with the pattern being visible through viewing angle dependent variation in brightness and contrast.